Method for forming a coating on a stent

ABSTRACT

A stent has first and second members. The stent is supported by a mandrel in a first position such that the mandrel is in contact with the first member and the second member is spaced from the mandrel. A method for coating the stent includes spraying or drying the stent, placing the stent in a second position such that the first member is spaced from the mandrel and the second member is placed in contact with the mandrel, and spraying or drying the stent while the stent is supported by the mandrel in the second position.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a divisional of application Ser. No. 12/027,947,filed Feb. 7, 2008, now U.S. Pat. No. 8,367,150, which is acontinuation-in-part of application Ser. No. 11/764,006, filed on Jun.15, 2007, now U.S. Pat. No. 7,897,195, the entire contents of whichapplications are incorporated by reference for all purposes.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to methods and devices for coating stents.

2. Description of the State of the Art

This invention relates to radially expandable endoprostheses, that areadapted to be implanted in a bodily lumen. An “endoprosthesis”corresponds to an artificial device that is placed inside the body. A“lumen” refers to a cavity of a tubular organ such as a blood vessel. Astent is an example of such an endoprosthesis. Stents are generallycylindrically shaped devices that function to hold open and sometimesexpand a segment of a blood vessel or other anatomical lumen such asurinary tracts and bile ducts. Stents are often used in the treatment ofatherosclerotic stenosis in blood vessels. “Stenosis” refers to anarrowing or constriction of a bodily passage or orifice. In suchtreatments, stents reinforce body vessels and prevent restenosisfollowing angioplasty in the vascular system. “Restenosis” refers to thereoccurrence of stenosis in a blood vessel or heart valve after it hasbeen treated (as by balloon angioplasty, stenting, or valvuloplasty)with apparent success.

Stents are typically composed of scaffolding that includes a pattern ornetwork of interconnecting structural elements or struts, formed fromwires, tubes, or sheets of material rolled into a cylindrical shape.This scaffolding gets its name because it physically holds open and, ifdesired, expands the wall of the passageway. Typically, stents arecapable of being compressed or crimped onto a catheter so that they canbe delivered to and deployed at a treatment site. Delivery includesinserting the stent through small lumens using a catheter andtransporting it to the treatment site. Deployment includes expanding thestent to a larger diameter once it is at the desired location.Mechanical intervention with stents has reduced the rate of restenosisas compared to balloon angioplasty. Yet, restenosis remains asignificant problem. When restenosis does occur in the stented segment,its treatment can be challenging, as clinical options are more limitedthan for those lesions that were treated solely with a balloon.

Stents are used not only for scaffholding but also as vehicles forproviding biological therapy. Biological therapy uses medicated stentsto locally administer a therapeutic substance. To reach effectiveconcentrations at the treated site via systemic drug administrationoften produces adverse or even toxic side effects. Local delivery is atreatment method because it administers smaller total medication levelsthan systemic methods and the drug is delivered to a specific site.Local delivery thus produces fewer side effects and achieves betterresults.

A medicated stent may be fabricated by coating the surface of a stentwith an active agent or an active agent and a polymeric carrier. Thoseof ordinary skill in the art fabricate coatings by applying a polymer,or a blend of polymers, to the stent using well-known techniques. Such acoating composition may include a polymer solution and an active agentdispersed in the solution. The composition may be applied to the stentby immersing the stent in the composition or by spraying the compositiononto the stent using various kinds of apparatus. The solvent thenevaporates, leaving on the stent surfaces a polymer coating impregnatedwith the drug or active agent.

The accuracy of drug loading, the uniformity of the drug distribution,stent coating quality, and coating material selection are criticalfactors in making the drug eluting stent. Having a robust and costeffective drug eluting stent manufacturing process to enable goodcoating quality, high throughput, high yield, low machine down time isan important goal for coated stent manufacturers.

SUMMARY OF THE INVENTION

Embodiments of the invention are directed to improving the coatingquality for a medical device coated with a substance, such as a polymercontaining a therapeutic drug. In one embodiment, a stent fixtureincludes a collet including a first portion and a second portionangularly spaced from the first portion with respect to an axis ofrotation for the collet, and a rotor coupled to the collet andconfigured to rotate the first and second portions between a first stentsupporting position and second stent supporting position, wherein thefirst and second stent supporting positions are separated by an angle ofless than 360°.

In another embodiment an apparatus for reducing coating defects on astent while a plurality of coats are applied to the stent includes afixture having a rotation axis and adapted to support a stent, and acontroller configured to rotate the fixture relative to the stent suchthat the stent is removed from the fixture between coats.

In another embodiment a stent fixture includes a member including firstand second portions extending radially outward from the axis, eachportion having a surface such that the respective first and secondsurfaces together form a part of a cone or ellipsoid. The portions maybe collets.

In another embodiment, a method for coating a stent having first andsecond members, the stent being supported by a mandrel in a firstposition such that the mandrel is in contact with the first member andthe second member is spaced from the mandrel, includes the steps ofspraying or drying the stent, placing the stent in a second positionsuch that the first member is spaced from the mandrel and the secondmember is placed in contact with the mandrel, and spraying or drying thestent while the stent is supported by the mandrel in a second position.The placing the stent in a second position may include the step ofrotating the mandrel relative to the stent.

In another embodiment, a method for coating a stent supported by acollet, the collet having an axis of rotation and stent supportingsurfaces separated by a first angle includes the steps of spraying thestent, rotating the collet and stent at different angular rates, andafter the stent and collet are rotated, spraying the stent. This methodmay be performed such that the rotating of the collet and stent atdifferent angularly rates includes rotating the collet relative to thestent rotation such that the collet rotates through an angle of betweenabout 45° to 90°, 90° to 120°, or 120° to 150° relative to the stent.

In another embodiment a stent fixture, such as a mandrel, is subject toa vibration or gas to dislodge a stent from the mandrel between sprayand coating cycles.

In another embodiment, a stent is loosely mounted on a mandrel, ormounted in an offset manner in order to reduce coating defects. Thestent may be mounted so that its longitudinal axis is offset from amandrel axis, or so that a predetermined gap is formed, as opposed topinching a stent between ends of a mandrel.

In another embodiment, collet has a shape of a cone, a portion of acone, or an ellipse. The collet may have notches, grooves or channels.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts a side view of a cylindrically-shaped stent.

FIG. 2 depicts a perspective view of a device used to coat stents.

FIG. 3 depicts a perspective view of a spraying portion of the device ofFIG. 2.

FIGS. 4A and 4B depict perspective views of a drying portion of thespray device of FIG. 2.

FIGS. 5A, 5B and 5C depict perspective views of a drying portion of thedevice of FIG. 2, including a mandrel assembly, spindle and supportbracket for the mandrel assembly.

FIG. 6 depicts a side view of the mandrel assembly in FIGS. 5A-5C, whichhas collets according to a first embodiment.

FIGS. 7A, 7B and 7C depicts a mechanism for restraining stent rotationbetween spray cycles.

FIGS. 8A and 8B depict features of a second embodiment of collets forthe stent mandrel assembly of FIG. 6.

FIG. 9 illustrates a stent support system for controlling a mandrelassembly during and between spraying and drying cycles.

FIG. 10 depicts a frontal view of a stent supporting end of a colletaccording to the second embodiment.

FIGS. 11A and 11B depict the frontal view of FIG. 10 with the stentsupporting end having channels formed thereon, and a detailed view of aportion of the stent supporting end.

FIGS. 12A, 12B and 12C show a planer and perspective views for a firststent type.

FIGS. 13A and 13B show a planer view, and a portion of the planer viewfor a second stent type.

FIGS. 14A and 14B depict the stent crowns at a “W” end and a “U” end ofthe stent of FIG. 12A relative to the portions of the collet of FIG. 11Awhen the stent is supported on a mandrel assembly according to thesecond embodiment.

FIGS. 15A and 15B depict the location of stent crowns at a “W” end and a“U” end of the stent of FIG. 13A relative to the collet of FIG. 11A whenthe stent is supported on a mandrel assembly according to the secondembodiment.

FIGS. 16A, 16B, 16C, 16D and 16E depict perspective, frontal and sideviews of a third embodiment of a collet.

FIGS. 17A, 17B, and 17C show views of a stent that was coated whilesupported on collets according to the third embodiment.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments of the present invention relate to coating implantablemedical devices such as stents. In particular, the embodiments of thepresent invention relate to methods and devices for spray coating anddrying stents.

A stent may have virtually any structural pattern that is compatiblewith a bodily lumen in which it is implanted. Typically, a stent iscomposed of a pattern or network of circumferential and longitudinallyextending interconnecting structural elements or struts. In general, thestruts are arranged in patterns, which are designed to contact the lumenwalls of a vessel and to maintain vascular patency. A myriad of strutpatterns are known in the art for achieving particular design goals. Afew of the more important design characteristics of stents are radial orhoop strength, expansion ratio or coverage area, and longitudinalflexibility. Embodiments of the present invention are applicable tovirtually any stent design and are, therefore, not limited to anyparticular stent design or pattern. One embodiment of a stent patternmay include cylindrical rings composed of struts. The cylindrical ringsmay be connected by connecting struts.

In some embodiments, a stent may be formed from a tube by laser cuttingthe pattern of struts in the tube. The stent may also be formed by lasercutting a metallic or polymeric sheet, rolling the pattern into theshape of the cylindrical stent, and providing a longitudinal weld toform the stent. Other methods of forming stents are well known andinclude chemically etching a metallic or polymeric sheet and rolling andthen welding it to form the stent.

In other embodiments, a metallic or polymeric filament or wire may alsobe coiled to form the stent. Filaments of polymer may be extruded ormelt spun. These filaments can then be cut, formed into ring elements,welded closed, corrugated to form crowns, and then the crowns weldedtogether by heat or solvent to form the stent.

FIG. 1 illustrates a conventional stent 10 having a plurality of struts12. The plurality of struts 12 are radially expandable andinterconnected by connecting elements 14 that are disposed betweenadjacent struts 12, leaving lateral openings or gaps 16 between adjacentstruts 12. Struts 12 and connecting elements 14 define a tubular stentbody having an outer, tissue-contacting surface and an inner surface.

The cross-section of the struts in stent 10 may be rectangular- orcircular-shaped. The cross-section of struts is not limited to these,and therefore, other cross-sectional shapes are applicable withembodiments of the present invention. Furthermore, the pattern shouldnot be limited to what has been illustrated as other stent patterns areeasily applicable with embodiments of the present invention.

The surface of an implant (surface energy, composition, roughness, andtopography) plays a major role during the initial phases of thebiological response (such as protein adsorption, cellular adherence, aswell as chronic phases of responses) to the implant. So, having goodcoating quality of the drug eluting stents (DES) is an important factorin minimizing the biological adverse effect.

Spray coating has been widely used by drug eluting stent (DES)manufactures. The spray process is a relatively simple, fast, and costeffective way of applying coating over the stent. The typical set-upincludes a fixturing (mandrel/coil/collet) to hold the stent in place toenable the stent to rotate and translate under a spray nozzle (and aheat nozzle). The coating solution is delivered to the nozzle tip by ametering system and it gets atomized into small droplets by the air orother external supplied forces (ultrasound, electrical). The ideal caseis to have a coating process that can produce defect-free DES. Butdepending on the stent fixture design, it is inevitable that there willbe some compromised areas induced by the contacting of the stent to thefixture.

Features of a mandrel design that lead to good coating quality includesmall contact areas to minimize the contact defect, easy of use,maintaining stent and mandrel 1 to 1 rotation, and clean-ability. Acollet having a conical surface exhibits these traits. Collets havingless surface area than a cone can also provide desirable features and,in addition, further reduce coating defects and/or enhance coatingquality over a conical surface. Moreover, the surfaces may be designedaccording to the stent geometry to further improve coating quality andreduce defects, as will be understood from the following disclosure.

By removing part of the surface areas on a conical surface, but stillmaintaining a self centering capability for the stent, a collet can notonly still provide a good support to the stent but eliminate theunnecessary contact points of the stent to the mandrel. The stent-colletcontact points, e.g., only two or three at each end of the stent, can beprecisely moved to the next open peak, crown, edge, etc. by using amechanism to rotate the collet relative to the stent. Stent contactpoints during the previous spray cycle are now open for the next spraycycle.

As indicated above, a medicated coating on a stent may be fabricated byspraying a coating composition including polymer and drug on the stent.Spray coating a stent typically involves mounting or disposing a stenton a support, followed by spraying a coating material from a nozzle ontothe mounted stent.

A spray apparatus, such as EFD 780S spray device with VALVEMATE 7040control system (manufactured by EFD. Inc., East Providence, R.I.) can beused to apply a composition to a stent. An EFD 780S spray device is anair-assisted external mixing atomizer. The composition is atomized intosmall droplets by air and uniformly applied to the stent surfaces. Othertypes of coating applicators, including air-assisted internal mixingatomizers (such as IVEK SonicAir nozzle), ultrasonic applicators (suchas Accu-Mist nozzle or MicroMist nozzle from SonoTek Co. in Milton,N.Y.), or a drop dispensing device can be used for the application ofthe composition.

To facilitate uniform and complete coverage of the stent during theapplication of the composition, the stent can be rotated about thestent's central longitudinal axis. Rotation of the stent can be fromabout 1.0 rpm to about 1000 rpm, more narrowly from about 30 rpm toabout 200 rpm. By way of example, the stent can rotate at about 150 rpm.The stent can also be moved in a linear direction along the same axis.The stent can be moved at about 1 mm/second to about 30 mm/second, forexample about 6 mm/second, or for a minimum of at least two passes(i.e., back and forth past the spray nozzle). In other applications, thespray nozzle can be devised to translate over the stent. The stent isrotated at a desired speed underneath the nozzle.

A nozzle can deposit coating material onto a stent in the form of finedroplets. An atomization pressure of a sprayer can be maintained at arange of about 5 psi to about 30 psi. The droplet size depends onfactors such as viscosity of the solution, surface tension of thesolvent, solution feed rate, and atomization pressure. The flow rate ofthe composition from the spray nozzle can be from about 0.1 mg/second toabout 10.0 mg/second, for example about 1.0 mg/second. Only a smallpercentage of the composition that is delivered from the spray nozzle isultimately deposited on the stent depending on the transfer efficiencyof the spray setup. By way of example, when a composition is sprayed todeliver about 1 mg of solids, only about 100 micrograms or about 10% ofthe solids sprayed will likely be deposited on the stent. The solidpercent in the composition typically can range from 0.1 wt % to 15 wt %,for example about 5 wt %.

To reduce or eliminate coating defects in coated stents, excessivesolvent in the applied coating material is removed through an in-processdrying cycle. Excessive application of the polymer or excessive solventleft in the coating can cause coating defects such as pool web(excessive material accumulated between stent struts) due to the lack ofgood wettability of the coating droplets over a stent with a tightgeometry.

To improve the coating quality, the coating process can involve multiplerepetitions or cycles of spraying forming a plurality of layers. Arepetition can involve a single pass or multiple passes of moving aspray nozzle (or moving the stent). A pass means moving the nozzle fromone end (e.g., proximal end) to the other end (e.g., distal end) of astent. Each repetition can be, for example, about 0.5 second to about 20seconds, for example about 10 seconds in duration. The amount of drycoating applied by each repetition can be about 1 microgram/cm² (ofstent surface) to about 75 micrograms/cm², for example, less than about20 micrograms/cm².

As indicated above, the coating composition can include a polymer and adrug dissolved in a solvent. Each repetition can be followed byin-process drying involving removal of a significant amount of thesolvent(s). In an embodiment, there may be less than 5%, 3%, or morenarrowly, less than 1% of solvent remaining in the coating afterin-process drying between repetitions. When the coating process iscompleted, all or substantially all of the solvent may be removed fromthe coating material on the stent. Any suitable number of repetitions ofapplying the composition followed by removing the solvent(s) can beperformed to form a coating of a desired thickness or weight. Excessiveapplication of the coating material can, however, cause coating defects.

Embodiments of the present invention may be illustrated by reference tothe exemplary spray coating device 200 depicted in FIGS. 2, 3, 4 and 5which is described in greater detail in U.S. application Ser. No.11/764,006, herein incorporated by reference in its entirety for allpurposes. Device 200 is configured to process two stents simultaneously.However, device 200 can process only one stent if desired. Device 200has a spraying zone 202 and a drying zone 204, which enable coating ofone stent and drying of another stent simultaneously. Stent supportassemblies, mandrels 208 and 222 can be moved between spraying zone 202and drying zone 204 via a rotating drum to allow simultaneous sprayingof a stent on one stent support assembly and drying of another stent onanother stent support assembly.

Spraying zone 202 has a spray nozzle 206 that is mounted above movablestent support assembly 208. As depicted by an arrow 242, stent supportassembly 208 is rotated during the coating process. Spray nozzle 206 istranslatable along a y-direction, as shown by double-headed arrow 205,along the axis of stent support assembly 208. Spray nozzle 206 is alsomovable along an x-direction as shown by an arrow 207.

Spray nozzle 206 is dwelled in a nozzle holder 220 which is attached toa mounting bracket block 218. Mounting bracket block 218 is coupled to alinear slide that can control movement of nozzle holder 220 and spraynozzle 206 back and forth in the x-direction during the application ofthe coating material over the stent. Mounting bracket block 218 is alsocoupled to a sliding stage to enable nozzle holder 220 along with spraynozzle 206 to side shift back and forth in the x-direction (245/207) toa position over upper funnel 214A after a spray cycle is complete. Theside-shifting of nozzle holder 220 along with spray nozzle 206 clearsthe path in the spray zone to allow the drum 240 to rotate to advancethe stent at the drying zone 204 to the spraying zone 202 to receivecoating material.

Drying zone 204 includes a drying nozzle 224 that can be positioned overa movable mandrel assembly 222 for supporting a coated stent duringdrying. Mandrel assembly 222 is inserted into a spindle 228, whichrotates the mandrel assembly 222 during the drying process, as indicatedby an arrow 243. In some embodiments, the same motor may providerotational motion to stent support assemblies 208 and 222. Drying nozzle224 includes an electrical heater 230 to generate heated gas for dryingnozzle 224. Drying nozzle 224 is movable and can shift in anx-direction, as shown by a double-headed arrow 245, from its positionshown in FIG. 2 to a drying position over mandrel assembly 222. Dryingnozzle 224 can be positioned above mandrel assembly 222 so that it candry a stent coated in spraying zone 202 by blowing warm gas over afreshly coated stent. Stent grippers 250 and 252 for clocking a stent,as described in detail below, are disposed below mandrel assembly 222.Heater 230 is movable in the x-direction as indicated by 502 (see FIG.4A).

Side shifting of drying nozzle 224 and spray nozzle 206 may beaccomplished with pneumatic slides or motor driven linear slides. Thisside-shift allows the indexing drum to rotate, and can also accommodatedifferences in the drying time and the spraying time. The side-shift ofdrying nozzle 224 to a deflection plate 508 of the drying air away fromthe stent to prevent over-drying while the other stent is finishing itsspray cycle.

Stent support assemblies 208 and 222 are supported at their distal endsby clamps 226 and 227, respectively. The proximal end of mandrelassembly 222 is shown supported by a spindle 228 in both the spray anddry zones. The proximal end of stent support assembly 208 is supportedin the same manner, but is hidden by spray nozzle 206. The spindle 228is mounted or coupled on a drum 240 which rotates as shown by arrow 232.Rotatable drum 240 can rotate to reverse the position of stent supportassemblies 208 and 222 so that stent support assembly 208 is in dryingzone 204 and mandrel assembly 222 is in the spray zone 202.

Referring to FIGS. 5A-5C, the jaw-like end support 227 providesalignment, support and facilitates automatic loading/unloading of amandrel and stent on the spindle 228. For example, a mandrel assembly222 having a finished stent 1123 mounted thereon can be removed from thedevice 200 by opening the jaws 1122, 1124 associated with support 227(FIG. 5B), then de-coupling collet 1104 from shank 1102. A new mandrelor support assembly 222 with stent is then placed in the device 200 bycoupling a proximal collet 1104 to shank 1102, placing pin end 1106 onlower jaw 1122, and then closing the jaws 1122 and 1124.

Referring again to FIG. 2, device 200 is designed to allow spraying ofstent in spray zone 202 while a coating layer previously applied atspray zone 202 is dried at drying zone 204. Simultaneous spraying anddrying reduces or eliminates idle time of sequential spraying and dryingoperation, thus increasing the throughput of a coating operation.

Specifically, a layer of coating material is applied to a first stentmounted on stent support assembly 208 by spray nozzle 206. At the sametime, a second stent mounted on mandrel assembly 222 with coatingmaterial already applied in spray zone 202 is dried by drying nozzle224. When both the spray coating on the first stent and drying of thesecond stent are completed, rotatable drum 240 rotates and positions thesecond stent (dried) at spray zone 202 and the first stent (freshlycoated) at drying zone 204. The first stent may then be dried at dryingzone 204 and a layer of coating material can be applied to the secondstent at spray zone 202. The spraying and drying can be repeated aselected number of times as necessary to obtain a desired coating masson each of the stents. Rotatable drum 240 can rotate clockwise orcounterclockwise to change the position of the first stent and secondstent between spray zone 202 and drying zone 204. Stent support assembly208 and stent mandrel assembly 222 are rotated in each spraying anddrying cycle. As shown by arrow 232, the first stent is rotated to sprayzone 202 and the second stent is rotated to drying zone 204, and afterthe spraying/drying cycle is complete the first stent is rotated back todrying zone 204 for drying the stent and the second stent is rotated tospray zone 202 to receive coating material.

FIG. 4B shows a slotted opening 510 through which warm gas passes fordrying a coated stent mounted on stent support mandrel 222. A deflectorshield 508 is positioned below drying nozzle 224 in its right-mostshifted position. Deflector shield 508 deflects the warm gas streamexiting drying nozzle 224 to the downstream evacuation when dryingnozzle 224 is shifted away from mandrel assembly 222 when the dryingcycle is complete. Perforated plates or screens 516 can be incorporatedinto drying nozzle 224 to improve the mixing of the hot gas exiting fromthe heating element (not shown) located at the upper portion of dryingnozzle 224 to provide an air stream with a uniform temperaturedistribution.

FIGS. 5A-C depict perspective views of a mandrel assembly 222 forsupporting a stent 1123, the supporting equipment for rotating themandrel assembly 222, and removing/replacing mandrel assemblies indevice 200. Collet 1104 is coupled at one end to a shank 1102, which isconnected in rotation to spindle or end cap 228. Spindle 228 is adaptedfor communicating rotational motion through shank 1102 to mandrelassembly 222. The rotation imparted to mandrel assembly 222 throughspindle 228 is depicted in FIG. 5B by an arrow 1103A, and the doublearrow in FIG. 6 (rotation axis “A”). This rotational motion occursduring the stent spraying and/or drying process and may also occurbetween spraying and drying cycles to reduce coating defects on thestent, as discussed below.

Referring to FIG. 5B, an end 1106A of the mandrel assembly pin 1106 isheld by a tailstock support 1112 during spraying and drying. Tailstocksupport 1112 is a jaw-like mechanism with two movable flat extensionarms 1114 and 1116 that can open as shown by arrows 1118 and 1120,respectively. Flat extension arms 1122 and 1124 with opposing wedge- orv-shaped cut-out sections are coupled to ends of movable arms 1114 and1116. Support fixture 1130 is composed of two end plates 1132 (outer)and 1134 (inner) that are used to house and support flat extension arms1114 and 1116. Proximal ends of extension arms 1114 and 1116 areconnected to two bars (one at an upper location and one at a lowerlocation) which are linked to an air cylinder to pull them up or down toclose or open the tailstock support. End 1106A of pin 1106 is heldbetween jaws 1138 and 1140 formed by opposing wedge-shaped cut-outsections of plates 1122 and 1124. In this action, end 1106 a may beadequately supported to prevent excessive run-out when spindle 228 isrotated. Further, this mechanism permits the stent 1123 to be easilydismounted by separating jaws 1138 and 1140 and decoupling the stentsupporting assembly 222 from the shank 1102. A finished stent 1123 andmandrel assembly 222 can then be removed from the device 200.

As mentioned earlier, it is desirable to reduce or perhaps eliminatecoating defects that can result from stent contact with supports, suchas mandrels, during coating. While some coating defects can be minimizedby adjusting the coating parameters, other defects occur due to thenature of the interface between the stent and the apparatus on which thestent is supported during the coating process. Surface contact betweenthe stent and the supporting apparatus can create regions in which theliquid composition can flow, wick, and collect as the composition isapplied. As the solvent evaporates, the excess composition hardens toform excess coating at and around the contact points between the stentand the supporting apparatus. Upon the removal of the coated stent fromthe supporting apparatus, the excess coating may stick to the apparatus,thereby removing some of the needed coating from the stent and leavingbare areas. Alternatively, the excess coating may stick to the stent,thereby leaving excess coating as clumps or pools on the struts orwebbing between the struts. Thus, it is desirable to minimize theinfluence of the interface between the stent and the supportingapparatus during the coating process to reduce or eliminate coatingdefects.

Coating defects associated with the stent-mandrel contact points can beminimized by addressing the manner in which the stent is mounted to amandrel, the design of the mandrel collets and the manipulation of thestent and/or mandrel between the coating and drying cycles discussedearlier. These methods and apparatus will now be discussed in greaterdetail by reference to embodiments of mandrel assembly 222.

Mandrel assembly 222 according to one embodiment is shown in side viewin FIG. 6. Proximal collet 1104 and distal collet 1105 haveconical-like, or tapered cylinder surfaces 1104A and 1105A,respectively, which support proximal and distal ends of the stent 1123on mandrel assembly 222. Stent 1123 is supported by collets 1104 and1105 such that at least a portion of the surface 1104A and 1105A isreceived within the bore of the stent 1123. Collets 1104, 1105 areconnected through a pin 1106 which can be slidingly received through abore formed in collet 1105 and fixed at one of its ends to collet 1104.The pin 1106 has a diameter less than the stent 1123. For example, thepin 1106 has a diameter between about 0.010″ and 0.030″. Pin 1106 can bemade of a metallic material such as Nitonol wire or other suitablematerial that provides adequate bending and torsional stiffnessproperties. As implied above, collets 1104, 1105, pin 1106 and the stent1123 are pre-assembled before being placed into device 200.

Collet surfaces 1104A and 1105A can have a roughened surface to absorbexcess coating material, which can reduce coating defects. For example,a micro-blasting tool can be applied to the surfaces 1104A, 1105A tocreate a micro depot surface to help spread out, or channel awayoverspray. The finish of the surface can be controlled by the type ofbead, duration of treatment, and pressure of the air supplied.

In some embodiments, stent 1123 is gently pinched between collets 1104,1105 and in other embodiments a predetermined gap, or lose contact ismade between the stent and collets when the mandrel assembly 222 isassembled. In some embodiments a gap or lose contact will reduce coatingdefects by a significant amount. When a stent is pinched betweencollets, the surface-to-surface contact between the stent and thecollets is greater then when a lose contact is made. In addition,surfaces are brought closer together. Hence, with increased surfacecontact and surfaces brought closer together, the coating material willhave more of a tendency to buildup between or near the surfaces due towicking, thereby leading to undesirable webbing or bridges formed overthe stent, or even bare spots on the stent surface.

During assembly, a gap or lose fit may be achieved as follows. First,collet 1104 with pin 1106 is placed vertical and stent 1123 dropped downover pin 1106 so that it rests on collet surface 1104A. The distalcollet 1105 is then inserted onto pin 1106 and moved down until thecollet 1105 is barely touching the distal end of the stent 1123. Thisgap may be a few thousands of an inch. Further, it has been found thatwhen there is only a lose contact between the stent 1123 and mandrelassembly 222, as opposed to pinching the stent between the collets 1104,1105, the mandrel assembly still achieves a one-to-one (“1:1”) rotationbetween the stent 1123 and the collets 1104, 1105. That is, there is noslippage of the stent on the mandrel assembly 222 during spraying anddrying cycles. Further, with this lose fitting, the stent 1123 can stillbe properly aligned so that the stent 1123 will not touch the pin 1106to minimize any stent inner diameter defects.

In some embodiments, a stent may be periodically separated from thecollet surfaces during a spraying or coating cycle as the mandrelassembly 222 rotates. With the stent 1123 periodically separated fromthe collets, there can be less buildup of material and, therefore, areduction in coating defects. In some embodiments, an ultrasonicvibration is periodically supplied to the pin 1106, resulting in avibration of the collets which can break any adhesion between the stent1123 and collet 1104, 1105 contact points caused by the sprayed coatingmaterial. The same break-up may be achieved by periodically supplying apulse or puff of air to the stent, thereby temporarily dislodging thestent from the collet surfaces. In some embodiments, the stent is loadedonto the collets 1104, 1105 such that the stent longitudinal axis isoffset from the rotation axis A of the mandrel assembly 222 (FIG. 6). Inthese embodiments, the stent 1123 will separate from collets (displacinglaterally of axis A) as the mandrel assembly 222 rotates due to theoffset between the stent axis and axis A. This can also help to reduceany stent inner diameter defects.

In some embodiments the stent is restrained and the collets shifted backand forth (i.e., brief clockwise, followed by counterclockwiserotations) to break-up any adhesion between the stent and the collets.This process may be performed between each spraying and drying cycle.For example, this process may be performed when the mandrel assembly 222is positioned in the drying zone 204 and just following a drying cycle.The rotational motion may be supplied by the spindle 228. As shown inFIG. 2 and in FIGS. 7A-7C, stent gripper plates 250 and 252 provide amechanism to steadily hold the stent while the mandrel assembly 222 isrotated so as to change or shift contact points between the stent andthe collets, or simply to break-up any adhesion between the stent andthe collets. FIG. 7A depicts a close-up view of stent gripper plates 250and 252 from FIG. 2, which are positioned below the stent mandrelassembly 222 (as depicted by stent 1123 and pin 1106). Stent gripperplates 250 and 252 are disposed at a distance H from one anotherinitially, the distance H being greater than the outside diameter of astent that is being coated. Plates 250 and 252 can be shifted towardeach other as shown by arrows 1206 and 1208 and upwards 1210, 1212towards the stent 1123 between spraying cycles.

Upon drying of a stent, stent gripper plates 250 and 252 are firstshifted upwards towards the mandrel assembly 222 so that stent 1123 isbetween plates 250 and 252, as depicted in FIG. 7B. Stent gripper plates250 and 252 then move towards one another as shown by arrows 1206 and1208. As depicted in FIG. 7C, stent grippers 250 and 252 move close toeach other to form a predetermined gap suited to gently hold stent 1123while mandrel assembly 222 is rotated relative to stent 1123, asrepresented by arrow 1214 in FIG. 7C. The mandrel assembly 222 can berotated or clocked just enough to move any contact points between stent1123 and any part of the mandrel assembly, for example, less than 5°.Alternatively, the mandrel assembly 222 can be rotated greater than 5°,10°, 30°, 60°, 90°, 270°, or greater than 360° between each or some ofthe spraying cycles. In addition, the mandrel assembly can be rotatedclockwise or counter-clockwise. The rotating or clocking can beuni-directional or bi-directional. For example, the mandrel assembly canbe clocked back and forth one or more times.

In some embodiments the mandrel assembly 222 may have collets withcollet surfaces that take a form intended to place less of the stent incontact with the collet, that is, less than the surface contact betweena stent and a tapered cylinder or truncated cone as in the case ofcollets 1104 and 1105. FIG. 8A depicts a perspective view of a collet600 according to embodiments that include a stent contacting end 602,which has stent-contacting surfaces 608 distributed over, or lyingwithin an imaginary surface, such as a truncated cone, or taperedcylinder imaginary surface. Collet surfaces 608 contact less of thestent than collet surface 1105A. In one sense, collet 600 may be viewedas a collet with conical surfaces 608 but with portions of the coneremoved so that surfaces 608 have only a portion of surfaces 1104A and1105A to contact the stent. As such collet 600 may be formed from acollet 1104 with grooves, notches or depressions formed on thestent-contacting surface 1104A.

Collet 600 includes an end 604 that may be used to grip collet 600 by atool, such as a tool used to place collet 600 over the pin 1106, orremove the collet 600 from the pin 1106. A bore having an opening 606for receiving or withdrawing the pin 1106 from the collet 600 when astent is mounted on, or dismounted from the pin 1106 is provided at thegeometric center, or axis of rotation A of the mandrel assembly 222. Inthe following discussion, collet 600 shares the same properties as, andmay be used in device 200 in the same manner as collet 1105 except wherethe following discussion makes apparent the distinctions between collet1105 and collet 600. Collet 600 may be used at both the proximal end(collet 600) and distal end (collet 600′) of the mandrel assembly 222.Unless otherwise specified, it will be understood that the featuresdiscussed earlier in connection with device 200 apply equally to adevice that uses collets 600 and 600′ in place of collets 1104 and 1105.

FIGS. 10 and 11 depict two embodiments of the stent supporting end 602of collet 600. For convenience, in the following discussion adescription of collet support 602 is sometimes made with respect tocylindrical-type coordinates (R,O,Z) with center at rotation center A.Thus, the axis A is the “Z” axis. End 602 may have three portions 610A,610B and 610C, each of which providing a surface 608A, 608B, 608C,respectively, intended to contact the stent such that at least a portionof the supporting end 602 is disposed within the stent bore when thestent is supported by collet 600. Portions 610A, 610B and 610C mayextend radially outward from hole 606 to provide surfaces 608. Eachsurface 608 may be rectangular with rounded edges and is placed incontact with one or more crowns or peaks of a stent.

In some embodiments members 610A and 610B are angularly spaced by anangle θ1, and members 610B and 610C by an angle θ2. In some embodiments,the angles θ1 and θ2 may be considered separation angles betweensurfaces 608. In some embodiments, members 610 may be equally spaced,such as by an angle 120° between each of the three members. In someembodiments there are more than three members 610, in other embodimentsless, such as in the case of collet end 702 shown in FIGS. 16A-D.

In some embodiments, each member 610 may include a groove, depression,channel or notch 620 extending radially outward from hole 606, asillustrated in FIG. 11A-B. The channels 620A, 620B and 620C bisect eachof the respective surfaces 608A, 608B, 608C so that the amount ofsurface area in contact with the stent is reduced, but with retaining a1:1 rotation ability during a spray and dry cycle. As mentioned above,surfaces 608 may lie within an imaginary tapered cylindrical or conicalsurface. Imaginary surface is intended to mean the geometric surfacecommon to all surfaces 608. Thus, with respect to the embodiments ofFIGS. 10, 11 the surfaces 608 may be described as lying on the outersurface of a tapered cylinder having a height 602A, outer radius 602Band taper angle 602C, as depicted in FIG. 8B. In some embodiments, theimaginary surface may be an ellipsoid, such as that described by anellipse “E”, as illustrated in FIGS. 16A-16E. Surfaces 608A, 608B, 608Ccover less than the entire tapered cylinder, or these surfaces are theportions of the conical surfaces 1104A, 1105A not removed from thecollets 1104, 1105. Surfaces 1104A and 1105A of Collets 1104, 1105 coverthe entire tapered cylinder.

Collets 600 and 600′ (see FIG. 8A) may be used to support stents havinga repeating pattern of crowns, peaks or bends associated with aproximal/distal end cylindrical segments of the stent. The peaks, crownsor bends may be formed by member(s) forming a sinusoidal, undulating,curvilinear etc. patterns at proximal and distal ends, i.e., those endsthat would come into contact with collet surfaces 608. Two such stents,506 and 509, are shown in plane view in FIGS. 12A and 13A, and a portionof those stents are shown in FIGS. 12B and 13B. The stent 506illustrated in FIGS. 12A and 12B has six crowns A6 through F6 at a “W”end and six crowns L6 through G6 at a “U” end. Crowns A6 through F6 arealso shown in the perspective view of FIG. 12C. The stent 509illustrated in FIGS. 13A and 13B has nine crowns A9 through I9 at a “W”end and nine crowns J9 through R9 at a “U” end. FIGS. 14A and 14B showthe contact points between surfaces 608 and crowns of stent 506 at thedistal end (FIG. 14A) and proximal end (FIG. 14B) when stent 506 issupported by collets 600 and 600′ as shown in FIG. 8B. Thus, asillustrated in FIGS. 14A-B, crowns F6, B6, D6 of the stent 506 areplaced in contact with surfaces 608B, 608C and 608A, respectively; andcrowns L6, J6, H6 of the stent 506 are placed in contact with surfaces608B, 608A and 608C, respectively. Similarly, FIGS. 15A-B depict thecontact between collet 600 and crowns of the stent 509. When stent 509is supported by collets 600 and 600′ (FIG. 8B), surface 608B is incontact with crowns A9 and B9, surface 608A is in contact with crowns D9and E9, and surface 608C is in contact with crowns H9 and G9. At thestent 509 “U” end, surface 608B is in contact with crown K9, surface608A is in contact with crown Q9, and surface 608C is in contact withcrowns N9.

In some embodiments, a stent mounted on collets 600 and 600′ may beperiodically held while collets 600, 600′ rotate so that the contactpoints between the stent and surfaces 608 are changed. Similarembodiments were discussed earlier in connection with the collets 1104and 1105. Following a drying or spraying step, the stent may be at leastpartially restrained in rotation, e.g., using grippers 252, 250, so asto cause a sufficient degree of slippage to occur between collets 600,600′ and stent surfaces when spindle 228 applies a rotation to mandrelassembly 222. This allows contact points to change during the coatingprocess, which can reduce coating defects. Since stent surfaces incontact, or near collet surfaces will be free of the collet in asubsequent spray cycle, buildup of coating material, which causesdefects, is minimized or effectively avoided because every surface issprayed at least once when free of the collet surface.

As the stent 506 or 509 is held and the supporting collets 600, 600′rotated in the direction 1214 (see FIG. 10), surfaces 608 may be movedfrom a first crown to a second, angularly adjacent crown, a first crownto a third crown, a first crown to a fourth crown, etc. after eachcoating is applied. In some embodiments, a collet is rotated only afraction of a crown width between coating cycles, e.g., so that thecollet is rotated through a small angle between coating cycles. In someembodiments, the collet may be rotated, 5, 10, 30, 50, 60, 180, 270, or120 degrees following a spray and coat cycle. For example, in FIGS.14A-B stent 506 is mounted on collets 600, 600′ such that surfaces 608are placed in contact with the “W” and “U” ends crowns as explainedearlier. Collets 600 and 600′ are then rotated (clockwise andcounterclockwise, respectively) 60 degrees so that surfaces 608B, 608Aand 608C are removed from the “u”-shape crowns F6, D6, B6 and placed incontact with the “w”-shape crowns E6, C6 and A6. Simultaneously, thesurfaces 608B, 608A, 608C are moved from crowns L6, J6, and H6 to crownsK6, 16 and G6 in FIG. 14B. Alternatively, collets 600, 600′ may berotated only 30 degrees between coatings, such that a crown is centeredover a member 610 during one spray cycle, partially over the member 610in the next spray cycle, and then free of the member 610 for the thirdspray cycle.

Referring to FIG. 11B, the surfaces 608, e.g., surface 608B, has a widthD, a height L and a channel width G. In some embodiments, the dimensionsof D and G may be selected based on a particular stent geometry, such asthe geometry of stent 506 and/or stent 509. In some embodiments, thesurfaces 608 and channels 620 have angular extents, i.e., D and G, thatprovide a minimum amount of supporting surface area needed to provide1:1 rotation and self-alignment of the stent, but without causingcrowns, peaks, edges, etc. of the stent to stick or get caught within achannel 620 or between successive surfaces 608 as the collet 600 rotatesrelative to the stent.

As the stent is held and the collets rotate, it is preferred to avoidany sudden changes in the surface-to-surface contact. For example, ifthe angle between successive crowns or peaks of the stent is greaterthen the width D, then the stent may become caught on collet 600 ascollet rotates because one or more members 610 becomes trapped betweensuccessful peaks or crowns in the stent. This may damage the stent.Similarly, if the width G of the channel 620 is about the width of, orlarger than a crown or peak, then the crown or peak of the stent maybecome trapped within the channel 620 as the collet is rotated to a newposition under the stent. Again, this can damage the stent. In oneembodiment, D is selected so that it is at least equal to the length X6in FIG. 12B, or at least twice the length of X9 in FIG. 13B. In someembodiments, G is selected so that it is smaller than the width of apeak or crown. This should avoid a tendency for the stent crowns to fallor key into a channel or between successive members when the collet isrotated relative to the stent. In some embodiments a gap is formedbetween the stent collets, as discussed earlier, rather than pinchingthe stent between collets as this may further reduce instances of thestent becoming caught between successive surfaces 608, or at leastenabling stent crowns to safely ride over changes in the collet surfacewithout damage to the stent. The ability for the stent crowns tonegotiate over changes in the collet surface may be improved by forminggradual changes in the collet surface. For example, the slopes 611B maygradually transition to surfaces 608 rather than abruptly, as in thecase of a right angle, so that crowns of the stent will have lesstendency to get caught on a corner of the collet supporting end 602 whenthe collet 600 is rotated.

Referring to FIGS. 16A-16E, in a third embodiment a collet 700 has astent supporting end 702 with a pair of opposing surfaces 708A, 708Bthat lie within an imaginary elliptical surface based on an ellipse E. Acollet 700 may be used for both proximal and distal collets of themandrel assembly 222 and may function in the same manner as earlierembodiments. According to the third embodiment, less surface area isused to support the stent. In contrast to the embodiments shown in FIGS.10, 11, the stent supporting end 702 has only two stent-contactingsurfaces 708A, 708B which are spaced about 180° from each other, asshown in FIG. 16D. Surfaces 708C, 708D, which extend essentially over alength equal to the major axis of the ellipse, are formed as curvedsurfaces that lie inside the surface of the ellipsoid. Side surfaces708C and 708D have a taper angle 703B that is less than the taper angle703A for the surfaces 708A and 708B that lie on the ellipsoid. Forexample, taper 703B may be 10 degrees and taper 703A may be 30 degrees.Collet end 702 may also include rounded edges 709 so that stent crownscan pass over the corners when collet 700 is rotated. In someembodiments, stent supporting end 702 further includes a notch 720 asshown in FIG. 16E. Like notches or channels described in connection withcollets 600 and 600′, further reduces surface area contact while alsoadequately supporting and self-centering stent when it is loaded onmandrel assembly 222. With regards to collet 700, it is preferred that agap or lose fit is made between the stent, e.g., stent 506 or 509, andthe collets 700, 700′, rather than pinching the stent between thecollets. This will ensure that collet 700 can be rotated relative to thestent between spray and dry cycles without causing damage to the stent.This type of fit will also reduce buildup of coating material, asdiscussed earlier.

Collet 700 may be rotated through different angles when the collet 700is repositioned with respect to the stent. For example, with referenceagain to FIG. 14A, and replacing collet 600 with collet 700, surface708A is in contact with “u”-shape crown F6 and surface 708B is incontact with “w”-shape crown C6. After a drying or spraying step iscomplete, the stent 506 may be held in rotation and collet 700 rotatedthrough an angle of 60° or 120° so as to remove surfaces from crowns C6and F6 to crowns B6 and F6, or D6 and A6, respectively, when collet isrotated counterclockwise in FIG. 15A. In some embodiments collet 700 maybe rotated through a range of angles, in increments of 5, 10, 20, 30,40, 60, 90 or more degrees as discussed earlier in connection withcollet 600.

Collets 600 or 700 can be made of metallic or polymeric (like UHMWPE,Fluorinated polymers like Teflon, PEEK, etc) materials. They could alsobe injection molded or machined. Metallic material may be preferred toprovide rigidity and machine-ability. Collets 600, 700 can be mademanufactured as disposable or they can be machined for several uses. Thedisclosed collets 600, 700 or variations thereof, in light of thisdisclosure, can be applied to coat any stent (metallic or polymer), andcan designed to coat other tubular shaped medical devices (implantablecoils, grafts, etc).

Returning again to FIGS. 5A, 5B, as mentioned earlier spindle 228 may beused to impart rotation to the mandrel assembly 222 during a sprayingand/or drying operation. Spindle may also serve to impart intermittentrotations to the assembly 222 while the stent 1123 is restrained bygrippers 250, 252 to break adhesion or re-position stent relative tocollets as described earlier in connection with collets 1104, 600 and700.

FIG. 9 illustrates a side view of mandrel assembly 222 according thatmay be used in connection with the second or third embodiments. FIG. 9depicts a schematic illustration of a control system 800 for controllingrotation of the mandrel assembly 222 during spraying and drying cycles,and also between spraying and drying. As discussed earlier, spindle 228is used to rotate the mandrel assembly 222 as the stent is placed in thespray and dry zones 202, 204 via rotating drum 248. Following a sprayand dry cycle, the mandrel assembly 222 is disposed in the drying zone204 and stationary. At this point, controller 801 initiates the gripperplates 252, 250 sequence to restrain the stent, e.g., stent 506 or stent509. With stent restrained, controller 801 then initiates a programmedrotation of the mandrel assembly 222 via a motor 802 coupled to spindle228. For example, the controller may cause spindle to rotate the mandrelassembly 222 back and forth to break up any adhesion between the stentand the collets, rotate through angles of 5, 10, 20, 30 or greaterangles so as to place new stent crowns on the collet surfaces, etc. asdiscussed earlier. The sequence of rotations applied to the mandrelassembly 222 may be initiated automatically, following every spray anddry cycle, or at user-selectable times. Input parameters, which maydepend on the type of stent (i.e., length, number of crowns, etc.), typeof collet (e.g., collet 1104, 600 or 700) and weight of coating applied,can be supplied via a user interface 803 before the process of sprayingand drying begins. Motor 802 may be any suitable motor 802A with a gearreduction 802B between it and mandrel assembly 222, or motor 802 maycorrespond to a stepper motor, which can provide highly preciserotational motion.

EXAMPLE

A mandrel having a pair of collets 700 (without a channel 702) supporteda 18 mm Xience V stents (medium design). The members or lobes of thecollets each had a width D of 0.039 inches to 0.046 inches. Thestent-contacting surfaces of the collet were treated with a COMCO MicroBlaster before assembly of the mandrel. The stent was then loaded on themandrel. The stent was mounted in such as manner as to allow relativerotation motion between coatings. The coating process included thefollowing parameters: IVEK pump rate at 190 microsteps; unidirectional35 deg clocking of the collet relative to the stent between coatings;drying nozzle temperature at 45 degrees Celsius (at stent's site) anddrying air flow (MKS) set a 113 liter/min; and set the total spray passat 30 and start coating cycles. FIGS. 17A-C are 400× magnificationimages of the stent for a coating weight of 600 micrograms. FIG. 17Ashows no detectable internal diameter defect. End ring defects were alsonegligible (FIGS. 17B-18C).

While particular embodiments of the present invention have been shownand described, it will be obvious to those skilled in the art thatchanges and modifications can be made without departing from thisinvention in its broader aspects. Therefore, the appended claims are toencompass within their scope all such changes and modifications as fallwithin the true spirit and scope of this invention.

What is claimed is:
 1. A method for forming a coating on a stent havingfirst and second members, the stent being supported by a collet in afirst position such that a surface of the collet having a channel is incontact with the first member and such that the second member is spacedfrom and out of contact with the collet, the method comprising: sprayingor drying the stent while the stent is supported by the collet in thefirst position; followed by placing the stent in a second position suchthat the first member is spaced from and out of contact with the colletand such that the second member is placed in contact with the surface ofthe collet having the channel; and followed by spraying or drying thestent while the stent is supported by collet in the second position. 2.The method of claim 1, wherein the placing of the stent and the colletin the second position includes rotating the collet relative to thestent.
 3. The method of claim 1, wherein the first member is a firstcrown at an end of the stent, the second member is a second crown at theend of the stent, and the second member is immediately adjacent thefirst crown.
 4. The method of claim 1, wherein the surface of the collethaving the channel is one of a plurality of tapered surfaces whichtogether form a partial conical shape.
 5. The method of claim 1, furthercomprising periodically separating the stent from the collet byvibrating the collet or by supplying a pulse of air to the stent.
 6. Themethod of claim 5, wherein the separating of the stent from the colletis performed during performance of a spraying cycle while the stent isbeing rotated.